As many of you know, some of the best evidence for natural selection comes from viruses and bacteria, which always seem to be adapting to the newest vaccine. Well, a new study published in Science shows just how adaptable some of these little critters can be. As summarized in ScienceNews, an international team has sequenced the genomes from 240 individuals of a particularly nasty strain of pneumonia-causing bacteria (Streptococcus pneumoniae). These guys have been found to be resistant to multiple drugs, and this study shows why:

1. Since the strain’s emergence (estimated to be around 1970 or so), it has changed one of its DNA bases (called a “point mutation”) about once every 15 weeks.

2. This strain (just like other species of bacteria) swaps genetic material (called “recombination”) with other strains.   

Put these things together, and…voila! You get the relatively rapid introduction of new gene variants or, in some cases, entirely new genes. The most important ones seem to be genes that code for a sugar coating that makes the bacteria difficult for the immune system to neutralize. The irony here, of course, is that the driving force behind the evolution of this and other strains is the development of vaccines meant to identify these different sugar coatings. Rapid mutation rates and recombination in these bacterial strains allows their populations to come up with new variants that are immune to the latest vaccines. 

References

Croucher, N.J. et al. (2011). Rapid pneumococcal evolution in response to clinical interventions. Science 331: 430-434.

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